Electron bombardment ion source for a mass spectrometer having a nonspecular inner surface

ABSTRACT

An ion source for a mass spectrometer includes means defining an ionization chamber having a longitudinal axis, an electron source, and an anode electrode for accelerating electrons in a beam through the chamber along the axis. The ionization chamber is arranged in a geometrical configuration adapted for enhancing collisions with sample particles introduced to the chamber. The chamber is defined by a substantially continuous, cylindricallyshaped wall and includes a sample entrance aperture having a cross section equal to or less than the cross section of the electron beam. By this arrangement, the probability of collision between sample particles and the electron beam is increased and the number of ions thereby generated is substantially increased.

United States Patent [191 Delany et al.

1 ELECTRON BOMBARDMENT ION SOURCE FOR A MASS SPECTROMETER HAVING A NONSPECULAR INNER SURFACE [76] Inventors: Edward B. Delany, 70 Powder Horn Ln., Ridgefield, Conn. 06877; Ralph E. Mayo, 321 Crestview Cir., Media, Pa. 19063 [22] Filed: Feb. 28, 1972 [21] Appl. No.: 230,106

Related U.S. Application Data [63] Continuation of Ser. No. 47,667, June 19, 1970, abandoned, which is a continuation-in-part of Ser. No. 682,467, Nov. 13, 1967, abandoned.

[52] U.S. Cl 250/427, 250/288, 313/63 [51] Int.-Cl. l-l0lj 39/34 [58] Field of Search. 250/41.9 SB, 41.9 SE, 41.9 S;

[56] References Cited UNITED STATES PATENTS 2,848,620 8/1958 Backus 2 50/419 [111 3,812,367 [451 May 21, 1974 3,115,591 12/1963 Brunnee 250/41.9X

3,502,863 3/1970 Tsyama et a1 250/41.9 3,527,937 9/1970 Delany et al 250/4l.9

Primary Examiner-William F. Lindquist Attorney, Agent, or Firm-John K. Conant 5 7 ABSTRACT An ion source for a mass spectrometer includes means defining an ionization chamber having a longitudinal axis, an electron source, and an anode electrode for accelerating electrons in a beam through the chamber along the axis. The ionization chamber is arranged in a geometrical configuration adapted for enhancing collisions with sample particles introduced to the chamber. The chamber is defined by a substantially continuous, cylindrically-shaped wall and includes a sample entrance aperture having a cross section equal to or less than'the cross section of the electron beam. By thisarrangement, the probability of collision between sample particles and the electron beam is increased and the number of ions thereby generated is substantially increased.

4 Clairns, 8 Drawing Figures ELECTRON BOMBARDMENT ION SOURCE FOR A MASS SPECTROMETER HAVING A NONSPECULAR INNER SURFACE This application is a continuation of Ser. No. 47,667, filed June 19, 1970, now abandoned, which was a continuation-in-part of Ser.. No. 682,467, filed Nov. 13, 1967, now abandoned.

This invention relates to mass spectrometers. The invention relates more particularly to an improved ion source for a mass spectrometer.

A mass spectrometer includes an ion source adapted for forming ionized constituents of a vaporized sample under analysis by electron bombardment of the sample. The ions thereby generated are accelerated toward a collector electrode through a field which, during a scanning interval, varies in intensity in a manner for focusing ions of differing M/E ratios at an output aperture of the collector. Ions passing through this aperture excite an electron multiplier and an output signal thereof is coupled to a recording means for providing a record of the M/e spectrum.

In a known technique, the sample material is initially vaporized and introduced to the ion source through a leak chamber which leaks the vaporized sample into the ion chamber at a desired and generally relatively low flow rate. At a predetermined leak rate, the sensitivity of the mass spectrometer is dependent largely upon the efficiency with which the ion source creates ions. At low leak rates and in those instances when the sample quantity available for analysis is limited, the efficiency with which the ion source creates ions can be determinative of a meaningful analysis.

In particular, presently utilized ion sources include means for collimating a beam of the electrons into an intense, relatively narrow beam within an ionization chamber of the ion source. Sample molecules drift into the path of the beam and electron-molecule collisions occur. Ions are thereby created both by collision between electrons and sample molecules and by secondary collisions between ions, electrons, and molecules. In attempting to provide efficient ion creation, the dimensions of the ionization chamber are maintained relatively small and shaped for increasing the probability of electron, molecule, ion collisions. However, such practical limitations as the provision of means for establishing the collimating field and for introducing the sample into the ionization chamber have resulted in an ionization chamber which disadvantageously exhibits a relatively large dead volume and is accompanied by incomplete ionization of the sample. The sensitivity of the spectrometer is therefore substantially smaller than is theoretically attainable.

Accordingly, it is an object of this invention to provide an improved form of mass spectrometer.

Another object of the invention is to provide an improved ion source for a mass spectrometer.

A further object of the invention is to provide an ion source for a mass spectrometer adapted for substantially increasing the number of ions created from a vaporized sample at a predetermined leak rate.

Another object of the invention is to provide an ionization chamber adapted for increasing the number of particle collisions therein.

Still another object of the invention is to provide an ionization chamber adapted for increasing the probability of particles passing through the ionizing electron beam after wall collisions.

In accordance with this invention, an ion source for a mass spectrometer includes an ionization chamber having a longitudinal axis thereof, an electron source and an anode electrode for accelerating electrons in a beam through the chamber along the axis. The ionization chamber is arranged in a geometrical configuration adapted for enhancing collisions with sample particles introduced to the chamber. The chamber is defined by a substantially continuous, cylindricallyshaped wall and includes a sample entrance aperture having a cross section equal to or less than the cross section of the electron beam. Through this arrangement, the number of particle collisions and the number of ions thereby generated is substantially increased.

These and other objects and features of the invention will become apparent with reference to the following specifications and drawings wherein:

FIG. 1 is a diagram in block form illustrating the general arrangement of a mass spectrometer;

FIG. 2 is a perspective view of an ion source for use with the mass spectrometer of FIG. 1;

FIG. 3 is a perspective view of an ion source support plate illustrating one embodiment of the ionization chamber of the present invention;

FIG. 4 is a plan view, partly cut away and partly in section, illustrating the ion source of FIG. 2;

FIG. 5 is a sectional view taken along'lines 55 of FIG. 4;

FIG. 6 is a perspective view of an alternative embodiment of the ionization chamber of the present invention;

FIG. 7 is a sectional view taken along lines 77 of FIG. 6; and,

FIG. 8 is a sectional view of the ion source cylinder illustrating the relative dimensions of a cross section of the cylinder and of the electron beam.

The general arrangement of a mass spectrometer incorporating the ion source of the present invention will be described briefly with reference to FIG. 1. An ion source 10 is adapted for receiving vaporized gaseous or liquid samples from a reservoir and leak chamber 12 and for receiving a vaporized solid sample from a solid sample injection probe 14. A sample injection arrangement of this type is described and claimed in copending US. patent application Ser. No. 630,108, filed April 1 l, 1967, and assigned to the assignee of the present invention, now abandoned. The ion source 10, which is maintained in an evacuated environment, creates ions from the vaporized sample by bombarding molecules of the sample with an electron beam. The ions thereby created are accelerated by virtue of an electric potential to an ion target electrode, not illustrated. These accelerated ions traverse a confined path defined by a tubulation l6 and pass through an aperture in the target electrode to excite an electron multiplier 18. An electromagnet 20 and a source of scanning current 22 establish a time-varying magnetic field in the path of the ions and, during a scanning interval, cause ions of differing mass to charge ratio (M/e) to focus on the output aperture in the target electrode. Output signal amplitude from the electron multiplier 18 is representative of the abundance of ions of a particular M/e. This electrical indication is coupled to a recording means 24 such as a strip chart recorder via an amplifier 26.

As indicated hereinbefore, it is advantageous to efficiently create a large number of ions at a predetermined sample leak rate. In accordance with a feature of this invention, the ion source includes an ionization chamber having a longitudinal axis 27 (FIG. 4) and geometrical configuration particularly adapted for enhancingthe creation of ions. In particular, the ionization chamber is defined by a substantially uninterrupted, cylindrically-shaped wall 38. An electron beam 28 of cross section A (FIG. 8) is accelerated from a source 46 (FIG. 4) to an anode electrode 48 along the longitudinal axis of the chamber. The wall 38 defines a cross section area A (FIG. 8) which does not exceed the cross sectional area of the beam A, by a factor greater than 400, i.e., A 1'A 5 400. In addition cylindrical sample inlet apertures 39 (FIG. 5) to the chamber have a cross sectional area A which is equal to or less than the cross section of the beam A i..e., A /A, 24l.*(*When both the beam and the chamber have circular cross sections as shown in FIG. 8, the area factor will be the ratio of the square of the diameters (D /D 5 400.) This ion source arrangement, as confirmed by our operation of the ion source, results in a substantially enhanced creation of ions.

We believe the enhanced creation of ions is explainable by Knudsens Law of Cosines of molecular deflection and Claussings Law of Cosines relating to the flow of molecules from a small aperture into the chamber. Knudsens Law of Cosines of molecular deflection indicates a relatively high probability that molecules impinging upon a non-specular inner surface of the cylindrically-shaped wall will be deflected radially toward the longitudinal axis of the cylinder. Claussings Law of Cosines indicates a high probability that particles exiting from a small aperture will diverge from their general direction in accordance with a cosine function. The probability that a relatively large number of ions will be generated is increased with this arrangement, as compared with other known geometrical configurations for ionization chambers. Our experiments have confirmed that the creation of ions is substantially increased with this arrangement. When the ratio of A /A exceeds 400 or the ratio of A IA is greater than 1, then the effectiveness of the ion source is believed to be substantially decreased.

The ion source having an ionization chamber constructed in accordance with this invention is illustrated in detail in FIGS. 2-7. FIG. 2 illustrates the general arrangement of the ion source. An ion source of this general type is described and claimed in co-pending US. patent application Ser. No. 630,133, filed on Apr. 11, 1967, and which is assigned to the assignee of the present invention, now US. Pat. No. 3,527,937. The ion source includes a lower support plate 32 upon which is mounted a first plate 34 forming an electrostatic lens,

a second 'plate 36 forming an electrostatic lens, and a formed of stainless steel or other suitable material and are assembled by spot welding. Each of the plates 34 and 36 includes a lens aperture 40 while the body 38 includes apertures 39 located therein at a point along its length for the introduction of a vaporized sample from opposite sides of the body. The plate 34 along with a focusing grid 46 (discussed hereinafter) provide potentials for focusing an electron beam at an aperture in the plate 34 while the plate 36 along with the anode potential provide for focusing the beam exiting from the ionization chamber at the anode 48. The body 38 includes the slot 42 (FIG. 5). Ions formed within the chamber leave the chamber via this slot at a point thereof which is aligned with an aperture 44 in a central section of this plate 32. Preferably the inlet apertures 39 and slot 42 are located at or about the same axial position of the chamber.

An electron beam is established in the chamber by a source of electrons including a filament surrounded by a beam intensity control and focusing grid 46 (FlG. 4) and an anode electrode 48, each disposed at opposite ends of the body 38. Theseelectrodes are aligned with the apertures 40 in the electrostatic lens plates 34 and 36. The filament and the grid electrode are mounted on a header 50 and extend through a pole piece 52 of a.v

magnetic circuit while the anode 48 and an anode heater filament 54 are mounted on a header 56 which extends through a pole piece 58. Current flowing in the filament 54 will heat the anode 48 and cause material deposited on the anode to be driven off. A long operating life and more efficient operation is thereby provided for the anode.

A magnetic field extending in the axial direction of the body 38 is established by means comprising permanent magnets 60, 62, 64, and 66 along with the pole pieces 52 and 58. The magnetic field thereby established collimates an axial electronic beam into a relatively narrow beam during the passage of electrons through the ionization chamber. A pair of ferromagnetic steel inserts 68 and 70 are disposed between the magnets 60, 62, and 64, 66. Each of these inserts includes a channel aligned with the aperture 39 in the body 38 for introducing a vaporized sample into the chamber. A second stainless steel plate 72 having a plurality of tabs 73 with apertures located therein is spot welded to and encloses the assembly. These apertured tabs in the upper plate along with corresponding tabs in the lower plate 32 adapt the ion source 10 for ready removal from an evacuated ion source enclosure.

A gas or vaporized liquid sample is introduced to the ionization chamber from the reservoir and leak 12 by a capillary tubulation 74 extending through the channel in the insert 68 and through the aligned aperture in the wall of the member 38. Similarly, a volatilized solid sample is introduced to the ionization chamber by the solid sample probe 14 through a channel in the insert 70 and the other aperture in the chamber wall 38. The ion source assembly thus includes a cylindrical ionization chamber advantageously enhancing the creation of ions.

FIG. 6 illustrates an alternative embodiment of the ionization chamber wherein the chamber is formed as a bore in a solid body 80. This arrangement is advantageous in that it reduces the dead volume existing between the body 38 of FIG. 3 and the magnetic circuit components and thereby substantially reducing leakage. The body is adapted to be utilized with the general ion source arrangement illustrated in FIG. 4. A cylindrical cavity is formed in the body 80 along its length. The body 80 is utilized with the magnetic circuit, electrostatic lenses, source of electrons, and mounting arrangements as described hereinbefore.

In experiments conducted with an ion source con structed in accordance with the second aforementioned patent application, we have measured an ion source efficiency of l X amps/torr. When the cylindrical ionization chamber of FIGS. 2-5 herein was utilized in the same arrangement and under substantially the same conditions, we have measured an ion source efficiency of 35 X 10 amps/torr. The efficiency of the ion source is thereby greatly enhanced.

An improved ion source has been described which advantageously enhances the creation of ions and provides a resulting increase in sensitivity for a mass spectrometer.

While I have illustrated and described a particular embodiment of my invention, it will be understood that various modifications may be made therein without departing from the spirit of the invention and the scope of the appended claims.

We claim:

1. In a mass spectrometer an ion source comprising in combination:

means defining a single elongated ionization chamber for the spectrometer, said chamber having a longitudinal axis and a substantially uninterrupted, generally cylindrical inner surface providing a cross section area A said chamber comprising the only volume of said ion source wherein ions are created by electron bombardment;

electron bombardment ionization means for providing a continuous flow of electrons in a beam from one end of said chamber to the opposite end of said chamber for creating ions by electron bombardment of a vaporized sample molecular material by said beam;

said ionization means including a source of electrons disposed at said one end of said chamber and an anode electrode disposed at said opposite end of said chamber for accelerating electrons in said beam in an axial direction through said chamber;

magnetic circuit beam collimating means for restricting said electron beam to a cross sectional area A wherein the ratio of the areas AJA is greater than 1, but less than 400;

said chamber having a generally circular cross section and exhibiting a substantially nonspecular inner surface for reflecting radially a substantial number of vaporized sample molecules impinging thereon toward said longitudinal axis of said chamber for increasing the probability of collisions with said electron beam in accordance with the law of cosines; means including an aperture of cross sectional area A communicating with said chamber for introducing a vaporized'sample in small quantities thereto for bombardment by said electron beam thereby producing ions, and wherein the ratio of areas A /A is less than 1; and

means for removing said ions created by said electron bombardment ionization means from said chamber including an exit aperture.

2. The combination in accordance with claim 1 wherein said chamber is defined by an elongated tube and first and second electrostatic plates positioned at opposite ends of the tube and having an electron beam aperture located therein.

3. The combination of claim 2 wherein said tube includes first and second sample introducing apertures positioned at a same axial location for introducing a sample to said chamber.

4. The combination of claim 3 including a notch fonned in said tube, and an ion exit aperture plate positioned adjacent said notch and having an ion exit aperture formed therein, and said first and second sample introducing apertures and said ion exit aperture are positioned at a same axial location. 

1. In a mass spectrometer an ion source comprising in combination: means defining a single elongated ionization chamber for the spectrometer, said chamber having a longitudinal axis and a substantially uninterrupted, generally cylindrical inner surface providing a cross section area A2, said chamber comprising the only volume of said ion source wherein ions are created by electron bombardment; electron bombardment ionization means for providing a continuous flow of electrons in a beam from one end of said chamber to the opposite end of said chamber for creating ions by electron bombardment of a vaporized sample molecular material by said beam; said ionization means including a source of electrons disposed at said one end of said chamber and an anode electrode disposed at said opposite end of said chamber for accelerating electrons in said beam in an axial direction through said chamber; magnetic circuit beam collimating means for restricting said electron beam to a cross sectional area A1 wherein the ratio of the areas A2/A1 is greater than 1, but less than 400; said chamber having a generally circular cross section and exhibiting a substantially nonspecular inner surface for reflecting radially a substantial number of vaporized sample molecules impinging thereon toward said longitudinal axis of said chamber for increasing the probability of collisions with said electron beam in accordance with the law of cosines; means including an aperture of cross sectional area A3 communicating with said chamber for introducing a vaporized sample in small quantities thereto for bombardment by said electron beam thereby producing ions, and wherein the ratio of areas A3/A1 is less than 1; and means for removing said ions created by said electron bombardment ionization means from said chamber including an exit aperture.
 2. The combination in accordance with claim 1 wherein said chamber is defined by an elongated tube and first and second electrostatic plates positioned at opposite ends of the tube and having an electron beam aperture located therein.
 3. The combination of claim 2 wherein said tube includes first and second sample introducing apertures positioned at a same axial location for introducing a sample to said chamber.
 4. The combination of claim 3 including a notch formed in said tube, and an ion exit aperture plate positioned adjacent said notch and having an ion exit aperture formed therein, and said first and second sample introducing apertures and said ion exit aperture are positioned at a same axial location. 